The long-term goal of this project is to investigate the mechanisms by which endogenous murine mammary tumor virus (MuMTV) induces late-occurring mammary tumors at a low incidence in certain strains of mice. Our preliminary studies of some late-occurring mammary tumors from CH3f and Af mice revealed that the majority of these tumors contain at least one new endogenous provirus integrated at the same site in the cell genome, suggesting that the promoter insertion model proposed for avian leukemogenesis may be applicable to mammary tumorigenesis in strains of mice with a low incidence of mammary tumors. We plan to determine if this is so by examining the DNA of mammary tumors from Af and RIIIf mice for site-specific MuMTV proviral integration and for the presence of novel mRNAs containing proviral and cellular sequences using cloned cellular and MuMTV-specific probes. In order to understand what role integration sites may play in mammary tumor induction we will determine if the cellular sequences into which new proviral DNA is integrated are hypomethylated and expressed in the normal mammary glands. We will also determine the origin of the newly integrated proviruses in tumors using a combination of genetic and molecular biological approaches. Using sera from mice bearing BALB/c-ST cell-induced tumors, we have recently detected what appears to be a tumor specific andtigen (p86/S) in the BALB/c-ST cell line that we established from a BALB/c tumor. We also observed that the tumor-producing BALB/c-ST cells, when infected in vitro by MuMTV from RIII mice, become non-tumorigenic and the expression of p86/S is greatly reduced. We propose to study the properties of the p86/S protein using biochemical and immunological methods, identify, by cDNA cloning, the gene that encodes this protein and determine if MuMTV plays a role in the activation of the p86/S gene. In order to determine the significance of the p86/S protein in murine mammary tumorigenesis, we will examine normal mammary cells and spontaneous or chemically-induced mammary tumors from a number of mouse strains for its expression, using the techniques of Western blotting and radioimmunoassays. In addition, the transforming potential of the p86/S protein will be evaluated in DNA transfection assays using the cloned p86/S gene and/or BALB/c-ST cell DNA. We will examine the mechanisms by which RIII-MuMTV infection results in the loss of tumorigenicity of the BALB/c-ST cells and reduces or abrogates the synthesis of the p86/X protein. Finally, we will investigate the role of MuMTV in the induction of leukemia in DBA/2 mice.